1. Field of the Invention
The invention relates to a reactor, in particular a fluidized-bed reactor, and a process for preparing granular polysilicon.
2. Description of the Related Art
Fluidized-bed reactors are used, for example for preparing trichlorosilane (TCS) by reaction of metallurgical silicon with HCl at 350-400° C. TCS can likewise be produced from metallurgical silicon and STC/H2 (STC=silicon tetrachloride) in a fluidized-bed reactor.
Fluidized-bed reactors are also used for preparing polycrystalline silicon granules.
This is achieved by fluidization of silicon particles by means of a gas stream in a fluidized bed, with the bed being heated to high temperatures by means of a heating device. Introduction of a silicon-containing reaction gas results in a pyrolysis reaction on the hot particle surface. Here, elemental silicon deposits on the silicon particles and the diameter of the individual particles increases. Taking off particles which have increased in size at regular intervals and adding smaller silicon particles as seed particles enables the process to be operated continuously with all attendant advantages. Silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4), and mixtures of these gases with hydrogen have been described as silicon-containing feed gas. Such deposition processes and apparatuses for carrying them out are known, for example, from U.S. Pat. No. 4,786,477 A.
U.S. Pat. No. 5,382,412 A discloses a process for preparing polycrystalline silicon in a fluidized-bed reactor, in which silicon starting particles are fed into the reactor in order to form a bed of silicon particles; the reactor bed is divided into a reaction zone in which a gaseous or vaporized silicon source is deposited as silicon metal on the silicon particles at a reaction temperature, and a heating zone in which a fraction of the silicon particles is heated to above the reaction temperature; a reaction gas which comprises the silicon source is introduced into the reaction zone, as a result of which the silicon particles in the reaction zone are fluidized; a carrier gas is introduced into the heating zone, as a result of which the silicon particles in the heating zone are fluidized; the silicon particles in the heating zone are heated by introduction of microwave energy into the heating zone; the silicon particles in an upper region of the heating zone are mixed with silicon particles of the reaction zone, as a result of which heat is transferred from the heating zone into the reaction zone; and unreactive fluidizing gas and reaction by-product gases are removed from the reactor.
However, owing to the temperature-dependent injection behavior of microwaves into silicon and the dependence of the energy input on the geometry of the reactor and the microwave supply, energy introduction which is nonuniform over the area occurs when using such a reactor. Substantial overheating of individual silicon particles and sintering together of particles and also formation of relatively large particle agglomerates takes place in the fluidized bed. These silicon agglomerates are undesirable in the product and, owing to their poorer flow properties, interfere substantially with reactor operation. Likewise, particles adhere to the fluidized-bed wall and are sometimes heated to melting (T>1400° C.). The substantial overheating of particles in the direct vicinity of the waveguide connections also led to excessive thermal stressing of the fluidized-bed wall. Although the fluidization of the fluidized bed and thus the mixing behavior has an equalizing effect with respect to the temperature distribution in the fluidized bed, this is greatly dependent on the degree of fluidization. The higher the gas velocity, the greater the extent to which particles are mixed vertically and horizontally. However, an increase in the gas velocity to far above the loosening velocity always results in an increase in the energy input required since the fluidizing gas generally flows into the fluidized bed with a significantly lower temperature than the particles and heats up approximately to the temperature of the particles during flow through the fluidized bed.
U.S. Pat. No. 7,029,632 B2 discloses a fluidized-bed reactor having a pressure-rated shell, an inner reactor tube which transmits heat radiation, an inlet for silicon particles, a tubular inlet for introduction of a reaction gas which divides the fluidized bed into a heating zone and a reaction zone located above the heating zone, a gas distribution device for introduction of a fluidizing gas into the heating zone, an outlet for unreacted reaction gas, fluidizing gas and the gaseous or vaporized product of the reaction, an outlet for the product, a heating device and also an energy supply for the heating device, with it being proposed that the heating device is a radiation source for radiation of heat which is arranged in an annular fashion around the heating zone outside the inner reactor tube and without direct contact with the latter and is configured in such a way that it heats the silicon particles in the heating zone by means of heat radiation to such a temperature that the reaction temperature is established in the reaction zone. Here too, the heating zone and reaction zone are separated vertically. This makes it possible to heat the fluidized bed by heating methods other than microwaves since no deposition on the wall can occur in the heating zone because there is no silicon-containing gas present there. Heat radiation heating by means of flat heating elements is provided, and introduces the heat uniformly over the circumference of the fluidized bed and in a positionally defined manner.
The main part of the heat radiation penetrates the inner reactor tube which has a high transmission for the heat radiation emitted by the selected heater and is absorbed directly by the silicon particles which are in the direct vicinity of the wall in the heating zone. Thus, the silicon particles in the fluidized bed can be directly heated uniformly over the circumference of the heating zone. Only a small proportion of the heat radiation is absorbed by the reactor tube and heats the latter.
The heating device is, for example, made up of heating elements composed of doped silicon or graphite or silicon carbide, fused silica tube radiators, ceramic radiators or metal wire radiators. The heating device is most preferably a meandering slotted tube composed of graphite with an SiC surface coating, which is arranged standing or suspended on the electrode connections in the reactor.
Cooling the interior of the reactor after the deposition is complete and flushing it with an inert gas such as H2, N2, Ar, He or a mixture of these gases is known from U.S. Pat. No. 8,017,024 B2. The cooled silicon particles are subsequently taken out, the reactor is disassembled, the reactor tube is replaced by a new tube, the reactor is assembled again and silicon particles are introduced into the reactor tube. The silicon particles are subsequently heated up and a fresh deposition operation commences.
The reactors described in the prior art have the problem that the reactor tube, which usually consists of fused silica, is destroyed when taken out from the reactor. This can result in contamination of the granular polysilicon. According to U.S. Pat. No. 8,017,024 B2, this can be prevented by dispensing with the disassembly of the reactor and passing a corroding gas into the reactor in order to remove the wall deposit from the reactor tube, with the polysilicon granules being taken from the reactor before the corroding treatment.
WO 2008/018760 A1 discloses protective tubes for heating devices in a fluidized-bed reactor, where the heating devices are located within the protective tube. The protective tubes serve to prevent or minimize contamination of silicon particles by the heating devices.
WO 93/20933 A1 describes a susceptor which is installed between the reactor well and an inductor. The reactor wall is heated by radiation heating. The susceptor is electrically conductive and is heated by induction. In this way, a uniform temperature distribution of the reactor wall can be achieved. The susceptor consists of high-temperature-resistant, thermally conductive material, preferably of graphite. However, this susceptor acts as a radiation shield and makes the process uneconomical.
The objects of the invention were derived from these problems.